Centrifugal separators are well known for use within the lubrication systems of vehicle internal combustion engines as efficient means for removing very small particulate contaminants from the constantly recirculated liquid lubricant over a long period of operation. Such centrifugal separators are usually of the self-powered type, in which a separation rotor comprising a canister is supported for rotation about a rotor axis within a housing, the canister being supplied with liquid lubricant at elevated pressure along the axis and said liquid being forced from the base of the canister (or other peripheral wall) by way of substantially tangentially-directed jet reaction nozzles, the reaction to said ejection causing the rotor canister to spin at high speed about the axis and thereby cause solid particles to migrate from the liquid passing through the canister and agglomerate on the peripheral walls thereof. The jet or reaction nozzles are directed substantially tangentially with respect to the rotation axis, at least in a plane orthogonal to the axis, so that jets of liquid which leave the rotor canister are instantaneously tangential to the fast spinning rotor.
It will be appreciated that the efficiency of separation is dependant upon the rotation speed of the rotor canister and also the quantity of lubricant passed therethrough in a given time, that is in turn, dependant upon the pressure drop between supply (canister) and housing and the dimensions of the nozzles, and within the constraints of such nozzle dimensions/pressure drops providing sufficient torque to overcome resistance to commencement of, and continuation of, rotation. Therefore, efficient rotation is dependent upon maintaining a good pressure drop across the reaction nozzles, that is, with the atmosphere inside the housing at ambient atmospheric pressure, so that it is necessary to drain this ejected liquid from the housing by gravity. In particular, it is necessary for the liquid to drain from the housing at a rate exceeding that of its supply to the rotor canister so that the ejected liquid does not accumulate in the housing and rise to a level at which it contacts, and stalls, the rotor. Such a gravity drainage system is frequently provided by means of a drainage duct, opening to the floor of the housing, of such cross-sectional area that not only can the liquid drain at a rate in excess of its supply to the rotor, but also can simultaneously permit venting of the housing to facilitate the drainage.
It will also be appreciated that the rotor canister may also be impeded in rotation by droplets of lubricant emitted by the nozzles that splash from the surfaces within the housing.
In practice there is almost invariably a requirement for overall minimal dimensions consistent with achieving a desired degree of functional efficiency, which requirement is met by having the housing dimensions only marginally exceeding the rotor, and the housing side wall usually conforms to the cylindrical locus of the volume swept by the rotating rotor, that is, is circular in cross-section that defines said minimal clearance. It will be appreciated that each tangentially directed and rotating reaction jet tends to strike the wall at a glancing angle. Whilst this reduces the tendency to splash and impede rotation to some extent, but not completely (and for which reason the axes of the reaction nozzles may be declined towards the floor of the housing), the lubricant is incident on the wall and/or floor at such angle as to cause this liquid to circulate around the housing as it falls towards the floor thereof and the drain.
It has been found that in some designs the effect of the circulating liquid in conjunction with a rapidly circulating atmosphere due to the rotor creates a vortex effect whereby the liquid accumulates in a vortex circulating about the periphery of the housing rather than flowing efficiently into the duct, notwithstanding the cross-sectional area of the drainage duct, and the level of this spent liquid gradually rises until the liquid touches the rotor and causes it to stall. The effect of such stalling is to permit the liquid to cease circulating at speed and begin to drain property, whereupon the rotor can recommence spinning until circumstances again force it to stall. Therefore, it is difficult to know when, and for what percentage of the time, the rotor is actually spinning and performing useful centrifugal separation.
The creation of such a vortex in the drainage liquid may be exacerbated by the structure of the housing in respect of the shape of surfaces surrounding, and leading to, the drainage duct and the axle mount.
Patent specifications GB 2049494 and 2120134 describe a centrifugal separator in which the floor of a circularly cylindrical housing is in the form of a substantially conical funnel leading to a central, or axial, drainage duct. The centrifugal separation rotor comprises a canister mounted for rotation within the housing (from the side wall of which it is separated by only a small radial clearance) on an axle which is supported, with respect to the housing, raised above the floor by a cage or spider comprising an array of divergent legs separated circumferentially by drainage apertures for the liquid ejected from the rotor.
The above-mentioned GB 2049494 describes briefly the problem of liquid being driven up the side wall towards the rotor and suggests attachment of a small lip on the side wall of the housing as a barrier to this.
It has been found that such centrifugal separator design when used with modern engines and lubricants, which operate at higher temperatures and lower viscosities, and correspondingly increased throughput for similar pressures, results in a significantly increased tendency for the liquid leaving the rotor nozzles to accumulate within the housing contrary to what might be expected. The dynamics of such a vortex with liquids of various viscosities suggests that such a lip baffle would produce, for liquids and viscosities typical of that time, a temporary impedance over which the vortex would tend to creep, but for liquids of lower operating viscosities a greater effect in halting the climb of such a vortex, but this is not experienced in practice.
Viewed somewhat simplistically, a vortex of liquid with a higher viscosity tends to have a more uniform structure and surface on which the newly ejected liquid impinges and merges with uniform results, whereas such a vortex of liquid with a low viscosity is found to have local turbulence within the body of the vortex and out of its surface and a tendency for both the liquid of the vortex and liquid ejected from the rotor that is incident on the vortex to splash and froth in the proximity of the rotor to the extent that the presence per se of such vortex separately from its climbing to fill the gap between rotor canister and housing, is a source of impedance to canister rotation detracting from the separation efficiency.
Therefore it is considered that a simple baffle to impede climbing of a vortex is not appropriate.
It is an object of the present invention to provide a centrifugal separator, including a separation rotor that is supported above the housing floor by a cage, and vortex disruption means to prevent accumulation in the housing of liquid ejected from the rotor as a vortex, that is capable of operating more efficiently than hitherto.
According to the present invention a self-powered centrifugal separator comprises a housing defined by a cylindrical side wall and by a floor shaped to effect drainage of fluid from the housing into a drainage duct, a centrifugal separation rotor, supported with respect to the housing for rotation therein about a rotation axis, arranged to receive fluid at elevated pressure and eject it by way of substantially tangentially directed reaction nozzles into the housing, said rotor being supported spaced with respect to the floor of the housing by means of a cage having a central mounting region coupled to the rotor and a surrounding apertured drainage region extending to the housing, and vortex disruption means, operable to inhibit accumulation of ejected fluid within the housing, comprising at least one deflection vane extending over a part of the cage drainage region in the same direction as rotation of the rotor, said vane having a first edge extending substantially parallel to the housing side wall closer than the spacing between said side wall and separation rotor and inclined with respect to said rotation axis in the direction of rotor rotation, and having a second edge extending from the end of said first edge to the vicinity of the central mounting region.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings.